This invention relates to the field of integrated circuits in general; and, more particularly, to resistors and their manufacture in integrated circuits.
Resistors in integrated circuits (ICs) are subject to heating, commonly known as Joule heating or ohmic heating, resulting from electrical current flowing through the resistors. The amount of such heating is a function of the amount of electrical current and the thermal paths from the resistor bodies to the IC substrates.
A typical architecture for a IC resistor includes a resistor body formed on top of field oxide and contacts (of, e.g., tungsten, aluminum or copper) for connecting ends of the resistor body to metal interconnect lines (of, e.g., aluminum or copper). Resistors are located on field oxide to minimize capacitance relative to other IC elements such as the substrate. Some IC resistors have silicided bodies, wherein a layer of metal silicide (e.g., nickel silicide, cobalt silicide, titanium silicide or platinum silicide) is formed on a top surface of a resistor body formed of polycrystalline silicon (i.e., polysilicon). Other IC resistors have polysilicon bodies with silicided regions at each end to reduce electrical resistance to the contacts that connect to the metal interconnect lines. Still other IC resistors may have metal bodies.
Joule heating, which is predominantly generated in the resistor body, can undesirably accelerate degradation mechanisms which may cause the IC to malfunction or fail completely. In silicided body resistors, such degradation mechanisms include breakdown of the metal silicide into regions of silicon rich material and metal rich material (known as agglomeration), and void formations in the metal interconnect lines where they overlap the resistor end contacts. In polysilicon body resistors, they include redistribution of dopant, agglomeration of silicided regions (if present) and void formations. And, in metal body resistors, they include void formations in the resistor bodies themselves, as well as in the metal interconnect/contact overlap. Electromigration defects are also accelerated by Joule heating.
Joule heating is exacerbated by the fact that field oxide is typically comprised of silicon dioxide, which has a low thermal conductivity compared to the semiconductor material comprising the IC substrate and active regions (e.g., silicon or silicon-germanium). This results in a higher thermal impedance between the resistor body and the IC substrate than would be the case for a similar resistor formed on top of a thin oxide layer, such as a gate dielectric layer used in MOS transistors, over an active region of the IC substrate. A resistor with a higher thermal impedance to the IC substrate produces a higher temperature in the resistor body than a similar resistor with a lower thermal impedance to the IC substrate for an equal amount of Joule heating (measured in watts), and thus undesirably accelerates the above degradation mechanisms. Another disadvantage of Joule heating in resistors is that it can also thermally accelerate degradation mechanisms in nearby transistors or other components.